The oxidation of n-butene to maleic anhydride has been conducted in two fixed-bed reactors containing two different catalysts for the best yields. Combining the two catalysts into one reactor has not been considered to be advisable, because the second catalyst attacks the olefin in such a manner that by-products are formed. Thus, before the desired reaction could occur, undesirable by-products, rather than the desired maleic anhydride, are formed.
Various catalysts that are effective for the conversion of n-butene are well known. Representative patents that exemplify such catalysts include: U.S. Pat. Nos. 3,414,631 and 3,642,930.
The second catalysts that are used in the invention are also known. Catalysts that are representative of the second catalyst of the invention are represented by U.S. Pat. applications Ser. No. 177,105, filed Sept. 1, 1971 now U.S. Pat. No. 3,907,834; Ser. No. 250,660, filed May 5, 1972 now abandoned; and Japanese Patent No. 25,736/71.
The combination of two catalysts into one reactor in a fluid-bed oxidation of the olefin to the anhydride, however, is not known. Also, the substantial yields of maleic anhydride obtained by the present invention could not be expected from the art.